Stdlib.Scanf(3o) OCaml library Stdlib.Scanf(3o)
NAME
Stdlib.Scanf - no description
Module
Module Stdlib.Scanf
Documentation
Module Scanf
: (module Stdlib__Scanf)
Introduction
Functional input with format strings
The module Scanf provides formatted input functions or scanners.
The formatted input functions can read from any kind of input, includ-
ing strings, files, or anything that can return characters. The more
general source of characters is named a formatted input channel (or
scanning buffer) and has type Scanf.Scanning.in_channel . The more gen-
eral formatted input function reads from any scanning buffer and is
named bscanf .
Generally speaking, the formatted input functions have 3 arguments:
-the first argument is a source of characters for the input,
-the second argument is a format string that specifies the values to
read,
-the third argument is a receiver function that is applied to the val-
ues read.
Hence, a typical call to the formatted input function Scanf.bscanf is
bscanf ic fmt f , where:
- ic is a source of characters (typically a formatted input channel
with type Scanf.Scanning.in_channel ),
- fmt is a format string (the same format strings as those used to
print material with module Printf or Format ),
- f is a function that has as many arguments as the number of values to
read in the input according to fmt .
A simple example
As suggested above, the expression bscanf ic "%d" f reads a decimal in-
teger n from the source of characters ic and returns f n .
For instance,
-if we use stdin as the source of characters ( Scanf.Scanning.stdin is
the predefined formatted input channel that reads from standard input),
-if we define the receiver f as let f x = x + 1 ,
then bscanf Scanning.stdin "%d" f reads an integer n from the standard
input and returns f n (that is n + 1 ). Thus, if we evaluate bscanf
stdin "%d" f , and then enter 41 at the keyboard, the result we get is
42 .
Formatted input as a functional feature
The OCaml scanning facility is reminiscent of the corresponding C fea-
ture. However, it is also largely different, simpler, and yet more
powerful: the formatted input functions are higher-order functionals
and the parameter passing mechanism is just the regular function appli-
cation not the variable assignment based mechanism which is typical for
formatted input in imperative languages; the OCaml format strings also
feature useful additions to easily define complex tokens; as expected
within a functional programming language, the formatted input functions
also support polymorphism, in particular arbitrary interaction with
polymorphic user-defined scanners. Furthermore, the OCaml formatted in-
put facility is fully type-checked at compile time.
Formatted input channel
module Scanning : sig end
Type of formatted input functions
type ('a, 'b, 'c, 'd) scanner = ('a, Scanning.in_channel, 'b, 'c, 'a ->
'd, 'd) format6 -> 'c
The type of formatted input scanners: ('a, 'b, 'c, 'd) scanner is the
type of a formatted input function that reads from some formatted input
channel according to some format string; more precisely, if scan is
some formatted input function, then scan
ic fmt f applies f to all the arguments specified by format string
fmt , when scan has read those arguments from the Scanf.Scan-
ning.in_channel formatted input channel ic .
For instance, the Scanf.scanf function below has type ('a, 'b, 'c, 'd)
scanner , since it is a formatted input function that reads from
Scanf.Scanning.stdin : scanf fmt f applies f to the arguments specified
by fmt , reading those arguments from stdin as expected.
If the format fmt has some %r indications, the corresponding formatted
input functions must be provided before receiver function f . For in-
stance, if read_elem is an input function for values of type t , then
bscanf ic "%r;" read_elem f reads a value v of type t followed by a ';'
character, and returns f v .
Since 3.10.0
exception Scan_failure of string
When the input can not be read according to the format string specifi-
cation, formatted input functions typically raise exception Scan_fail-
ure .
The general formatted input function
val bscanf : Scanning.in_channel -> ('a, 'b, 'c, 'd) scanner
bscanf ic fmt r1 ... rN f reads characters from the Scanf.Scan-
ning.in_channel formatted input channel ic and converts them to values
according to format string fmt . As a final step, receiver function f
is applied to the values read and gives the result of the bscanf call.
For instance, if f is the function fun s i -> i + 1 , then Scanf.sscanf
"x= 1" "%s = %i" f returns 2 .
Arguments r1 to rN are user-defined input functions that read the argu-
ment corresponding to the %r conversions specified in the format
string.
Format string description
The format string is a character string which contains three types of
objects:
-plain characters, which are simply matched with the characters of the
input (with a special case for space and line feed, see Scanf.space ),
-conversion specifications, each of which causes reading and conversion
of one argument for the function f (see Scanf.conversion ),
-scanning indications to specify boundaries of tokens (see scanning
Scanf.indication ).
The space character in format strings
As mentioned above, a plain character in the format string is just
matched with the next character of the input; however, two characters
are special exceptions to this rule: the space character ( ' ' or ASCII
code 32) and the line feed character ( '\n' or ASCII code 10). A space
does not match a single space character, but any amount of 'whitespace'
in the input. More precisely, a space inside the format string matches
any number of tab, space, line feed and carriage return characters.
Similarly, a line feed character in the format string matches either a
single line feed or a carriage return followed by a line feed.
Matching any amount of whitespace, a space in the format string also
matches no amount of whitespace at all; hence, the call bscanf ib
"Price = %d $" (fun p -> p) succeeds and returns 1 when reading an
input with various whitespace in it, such as Price = 1 $ , Price = 1
$ , or even Price=1$ .
Conversion specifications in format strings
Conversion specifications consist in the % character, followed by an
optional flag, an optional field width, and followed by one or two con-
version characters.
The conversion characters and their meanings are:
- d : reads an optionally signed decimal integer ( 0-9 +).
- i : reads an optionally signed integer (usual input conventions for
decimal ( 0-9 +), hexadecimal ( 0x[0-9a-f]+ and 0X[0-9A-F]+ ), octal (
0o[0-7]+ ), and binary ( 0b[0-1]+ ) notations are understood).
- u : reads an unsigned decimal integer.
- x or X : reads an unsigned hexadecimal integer ( [0-9a-fA-F]+ ).
- o : reads an unsigned octal integer ( [0-7]+ ).
- s : reads a string argument that spreads as much as possible, until
the following bounding condition holds:
-a whitespace has been found (see Scanf.space ),
-a scanning indication (see scanning Scanf.indication ) has been en-
countered,
-the end-of-input has been reached.
Hence, this conversion always succeeds: it returns an empty string if
the bounding condition holds when the scan begins.
- S : reads a delimited string argument (delimiters and special escaped
characters follow the lexical conventions of OCaml).
- c : reads a single character. To test the current input character
without reading it, specify a null field width, i.e. use specification
%0c . Raise Invalid_argument , if the field width specification is
greater than 1.
- C : reads a single delimited character (delimiters and special es-
caped characters follow the lexical conventions of OCaml).
- f , e , E , g , G : reads an optionally signed floating-point number
in decimal notation, in the style dddd.ddd
e/E+-dd .
- h , H : reads an optionally signed floating-point number in hexadeci-
mal notation.
- F : reads a floating point number according to the lexical conven-
tions of OCaml (hence the decimal point is mandatory if the exponent
part is not mentioned).
- B : reads a boolean argument ( true or false ).
- b : reads a boolean argument (for backward compatibility; do not use
in new programs).
- ld , li , lu , lx , lX , lo : reads an int32 argument to the format
specified by the second letter for regular integers.
- nd , ni , nu , nx , nX , no : reads a nativeint argument to the for-
mat specified by the second letter for regular integers.
- Ld , Li , Lu , Lx , LX , Lo : reads an int64 argument to the format
specified by the second letter for regular integers.
- [ range ] : reads characters that matches one of the characters men-
tioned in the range of characters range (or not mentioned in it, if the
range starts with ^ ). Reads a string that can be empty, if the next
input character does not match the range. The set of characters from c1
to c2 (inclusively) is denoted by c1-c2 . Hence, %[0-9] returns a
string representing a decimal number or an empty string if no decimal
digit is found; similarly, %[0-9a-f] returns a string of hexadecimal
digits. If a closing bracket appears in a range, it must occur as the
first character of the range (or just after the ^ in case of range
negation); hence []] matches a ] character and [^]] matches any charac-
ter that is not ] . Use %% and %@ to include a % or a @ in a range.
- r : user-defined reader. Takes the next ri formatted input function
and applies it to the scanning buffer ib to read the next argument. The
input function ri must therefore have type Scanning.in_channel -> 'a
and the argument read has type 'a .
- { fmt %} : reads a format string argument. The format string read
must have the same type as the format string specification fmt . For
instance, "%{ %i %}" reads any format string that can read a value of
type int ; hence, if s is the string "fmt:\"number is %u\"" , then
Scanf.sscanf s "fmt: %{%i%}" succeeds and returns the format string
"number is %u" .
- ( fmt %) : scanning sub-format substitution. Reads a format string
rf in the input, then goes on scanning with rf instead of scanning with
fmt . The format string rf must have the same type as the format
string specification fmt that it replaces. For instance, "%( %i %)"
reads any format string that can read a value of type int . The con-
version returns the format string read rf , and then a value read using
rf . Hence, if s is the string "\"%4d\"1234.00" , then Scanf.sscanf s
"%(%i%)" (fun fmt i -> fmt, i) evaluates to ("%4d", 1234) . This be-
haviour is not mere format substitution, since the conversion returns
the format string read as additional argument. If you need pure format
substitution, use special flag _ to discard the extraneous argument:
conversion %_( fmt %) reads a format string rf and then behaves the
same as format string rf . Hence, if s is the string "\"%4d\"1234.00"
, then Scanf.sscanf s "%_(%i%)" is simply equivalent to Scanf.sscanf
"1234.00" "%4d" .
- l : returns the number of lines read so far.
- n : returns the number of characters read so far.
- N or L : returns the number of tokens read so far.
- ! : matches the end of input condition.
- % : matches one % character in the input.
- @ : matches one @ character in the input.
- , : does nothing.
Following the % character that introduces a conversion, there may be
the special flag _ : the conversion that follows occurs as usual, but
the resulting value is discarded. For instance, if f is the function
fun i -> i + 1 , and s is the string "x = 1" , then Scanf.sscanf s "%_s
= %i" f returns 2 .
The field width is composed of an optional integer literal indicating
the maximal width of the token to read. For instance, %6d reads an in-
teger, having at most 6 decimal digits; %4f reads a float with at most
4 characters; and %8[\000-\255] returns the next 8 characters (or all
the characters still available, if fewer than 8 characters are avail-
able in the input).
Notes:
-as mentioned above, a %s conversion always succeeds, even if there is
nothing to read in the input: in this case, it simply returns "" .
-in addition to the relevant digits, '_' characters may appear inside
numbers (this is reminiscent to the usual OCaml lexical conventions).
If stricter scanning is desired, use the range conversion facility in-
stead of the number conversions.
-the scanf facility is not intended for heavy duty lexical analysis and
parsing. If it appears not expressive enough for your needs, several
alternative exists: regular expressions (module Str ), stream parsers,
ocamllex -generated lexers, ocamlyacc -generated parsers.
Scanning indications in format strings
Scanning indications appear just after the string conversions %s and %[
range ] to delimit the end of the token. A scanning indication is in-
troduced by a @ character, followed by some plain character c . It
means that the string token should end just before the next matching c
(which is skipped). If no c character is encountered, the string token
spreads as much as possible. For instance, "%s@\t" reads a string up to
the next tab character or to the end of input. If a @ character appears
anywhere else in the format string, it is treated as a plain character.
Note:
-As usual in format strings, % and @ characters must be escaped using
%% and %@ ; this rule still holds within range specifications and scan-
ning indications. For instance, format "%s@%%" reads a string up to
the next % character, and format "%s@%@" reads a string up to the next
@ .
-The scanning indications introduce slight differences in the syntax of
Scanf format strings, compared to those used for the Printf module.
However, the scanning indications are similar to those used in the For-
mat module; hence, when producing formatted text to be scanned by
Scanf.bscanf , it is wise to use printing functions from the Format
module (or, if you need to use functions from Printf , banish or care-
fully double check the format strings that contain '@' characters).
Exceptions during scanning
Scanners may raise the following exceptions when the input cannot be
read according to the format string:
-Raise Scanf.Scan_failure if the input does not match the format.
-Raise Failure if a conversion to a number is not possible.
-Raise End_of_file if the end of input is encountered while some more
characters are needed to read the current conversion specification.
-Raise Invalid_argument if the format string is invalid.
Note:
-as a consequence, scanning a %s conversion never raises exception
End_of_file : if the end of input is reached the conversion succeeds
and simply returns the characters read so far, or "" if none were ever
read.
Specialised formatted input functions
val sscanf : string -> ('a, 'b, 'c, 'd) scanner
Same as Scanf.bscanf , but reads from the given string.
val scanf : ('a, 'b, 'c, 'd) scanner
Same as Scanf.bscanf , but reads from the predefined formatted input
channel Scanf.Scanning.stdin that is connected to stdin .
val kscanf : Scanning.in_channel -> (Scanning.in_channel -> exn -> 'd)
-> ('a, 'b, 'c, 'd) scanner
Same as Scanf.bscanf , but takes an additional function argument ef
that is called in case of error: if the scanning process or some con-
version fails, the scanning function aborts and calls the error han-
dling function ef with the formatted input channel and the exception
that aborted the scanning process as arguments.
val ksscanf : string -> (Scanning.in_channel -> exn -> 'd) -> ('a, 'b,
'c, 'd) scanner
Same as Scanf.kscanf but reads from the given string.
Since 4.02.0
Reading format strings from input
val bscanf_format : Scanning.in_channel -> ('a, 'b, 'c, 'd, 'e, 'f)
format6 -> (('a, 'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g
bscanf_format ic fmt f reads a format string token from the formatted
input channel ic , according to the given format string fmt , and ap-
plies f to the resulting format string value.
Since 3.09.0
Raises Scan_failure if the format string value read does not have the
same type as fmt .
val sscanf_format : string -> ('a, 'b, 'c, 'd, 'e, 'f) format6 -> (('a,
'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g
Same as Scanf.bscanf_format , but reads from the given string.
Since 3.09.0
val format_from_string : string -> ('a, 'b, 'c, 'd, 'e, 'f) format6 ->
('a, 'b, 'c, 'd, 'e, 'f) format6
format_from_string s fmt converts a string argument to a format string,
according to the given format string fmt .
Since 3.10.0
Raises Scan_failure if s , considered as a format string, does not have
the same type as fmt .
val unescaped : string -> string
unescaped s return a copy of s with escape sequences (according to the
lexical conventions of OCaml) replaced by their corresponding special
characters. More precisely, Scanf.unescaped has the following prop-
erty: for all string s , Scanf.unescaped (String.escaped s) = s .
Always return a copy of the argument, even if there is no escape se-
quence in the argument.
Since 4.00.0
Raises Scan_failure if s is not properly escaped (i.e. s has invalid
escape sequences or special characters that are not properly escaped).
For instance, Scanf.unescaped "\"" will fail.
Deprecated
val fscanf : in_channel -> ('a, 'b, 'c, 'd) scanner
Deprecated.
Scanf.fscanf is error prone and deprecated since 4.03.0.
This function violates the following invariant of the Scanf module: To
preserve scanning semantics, all scanning functions defined in Scanf
must read from a user defined Scanf.Scanning.in_channel formatted input
channel.
If you need to read from a in_channel input channel ic , simply define
a Scanf.Scanning.in_channel formatted input channel as in let ib =
Scanning.from_channel ic , then use Scanf.bscanf ib as usual.
val kfscanf : in_channel -> (Scanning.in_channel -> exn -> 'd) -> ('a,
'b, 'c, 'd) scanner
Deprecated.
Scanf.kfscanf is error prone and deprecated since 4.03.0.
OCamldoc 2023-02-12 Stdlib.Scanf(3o)
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